1. Field of the Invention
The present invention relates to a planar lightwave circuit (PLC) which can decrease the coupling loss between a planar lightwave circuit and an optical fiber or between planar lightwave circuits.
2. Description of the Related Art
It is predicted that the planar lightwave circuits will be used more and more as main parts which have important functions such as routing of an optical signal in superfast large-capacity optical communication systems from now on. Especially, it is required to construct a larger optical communication system as capacity required for communication increases. In order to realize the enlargement of the optical communication system, it is necessary to downsize the planar lightwave circuit and to allow connection between many planar lightwave circuits.
FIG. 1 is a schematic diagram of structure of a conventional planar lightwave circuit. As shown in FIG. 1, an input waveguide 112 is formed on a substrate 111. The input waveguide 112 is connected to a slab waveguide 113 formed on the substrate 111. One end of a plurality of arrayed waveguides 114 formed on the substrate 111 is connected to the slab waveguide 113. A half waveplate 115 which gets rid of polarization dependence is provided at some midpoint of the arrayed waveguides 114. The other end of the arrayed waveguides 114 is connected to a slab waveguide 116 formed on the substrate 111. A plurality of output waveguides formed on the substrate 111 are connected to the slab waveguide 116.
When an optical signal is entered into the planar lightwave circuit 110 from the input waveguide 112, the optical signal is entered into the arrayed waveguides 114 via the slab waveguide 113 and polarization dependence is dissolved by the half waveplate 115. In addition, the optical signal is demultiplexed into signals of various wavelengths in the slab waveguide 116 due to delay line of the arrayed waveguides 114 so that demultiplexed signals are output from the output waveguides 117.
In order to downsize the planar lightwave circuit 110, it is very effective to adopt a waveguide (which will be called a superhigh- waveguide) in which relative refractive index difference  is a high value which is larger than 1% where relative refractive index difference  is the ratio of difference between the refractive index ncore of the core and refractive index nclad of the cladding to the refractive index ncore of the core as represented by the following equation (1). The reason is that the higher the relative refractive index difference  is, the more completely the light is confined in the waveguide so that the waveguide can be used even when it is bent by a small bending radius.=(ncore−nclad)/ncore  (1)
However, there is a problem in that the coupling loss of the superhigh- waveguide is very large.
As shown in FIG. 2, when an optical signal which propagates through a core 120a in an optical fiber 120 enters into a core 112a of the input waveguide 112, attenuation of the optical signal, which is called the coupling loss, occurs. The coupling loss occurs when connecting different types of waveguides The coupling loss occurs due to difference of field distribution between the different types of waveguides, and the coupling loss accumulates as connecting points increases. For example, as for 1.5 μm wavelength which is used in an optical communication system, large coupling loss about 3.5 dB occurs between a single-mode optical fiber of core diameter 9 μm and a superhigh- waveguide about 5 μm per side.
In the planar lightwave circuit 110 used in the optical communication system, downsizing and decreasing of the coupling loss are mutually contradictory. That is, although the circuit can be downsized by increasing , the coupling loss increases. Therefore, construction of a practical system has limitations That is, it becomes difficult to enlarge capacity of transmission lines unless the coupling loss of the superhigh- waveguide is decreased, so that functions and scale of the optical communication system may be limited.
As a method for decreasing the coupling loss between the superhigh- waveguide and the optical fiber, use of a spotsize converter in which core width is narrowed toward an end face of a substrate is known as shown in FIG. 3 (for example, Japanese laid-open patent application No.63-280202).
There is an region in which a spotsize is widened when the core width is narrowed to some extent. Then, it becomes possible to decrease the coupling loss by adjusting the widened field distribution with that of an optical fiber.
However, it is known that the coupling loss for the narrow taper spotsize converter largely changes due to slight fabrication error of core width,  and the like. Thus, the narrow taper spotsize converter has not been in practical use.